A threat launch detection system is a system that detects a weapon that is being directed at a target, with the target typically containing the threat launch detection system. In response to detecting a weapon directed at the target, which will be referred to as a threat or event throughout the present description, the threat launch detection system typically takes countermeasures to prevent the weapon from impacting the target. For example, an airplane may include a threat launch detection system designed to detect missiles fired at the airplane. When the system detects a missile, the system typically takes appropriate countermeasures in an attempt to prevent the missile from impacting the airplane, such as transmitting a signal to “jam” the seeker of the missile.
Different types of targets, which may be referred to as military vehicles in the following description, face different types of threats. Airplanes as previously mentioned face the threat of guided missiles, which may be “heat seeking” or infrared (IR) guided or radar guided missiles. Such missiles include engines or rockets that propel the missile through the air towards the airplane. Such a rocket continually burns to propel the missile and threat launch detection systems in aircraft exploit this fact to detect such threats. Other types of military vehicles, such as helicopters and tanks, face different types of threats. For example, a tank faces the threats of being shot at by a rocket propelled grenade (RPG), a shell from another tank, or any of a variety of other antitank weapons.
Threats such as a shell from another tank or an RPG are examples of what are known as “short-burn”, “motorless,” or “post-burnout” threats. These threats are so named because the charge or engine utilized to propel the threat is active for only a very short time when compared to other types of threats such as guided missiles. In the following description, such threats will be referred to simply as “short-burn” threats. As a result of the different characteristics of different types of threats, threat launch detection systems must be capable of detecting the types of threats most likely to be encountered by the type of military vehicle containing the system or the type of vehicle the system is designed to protect.
To detect these various types of threats, conventional threat launch detection systems utilize sensors formed by a sensor array in combination with suitable optics that provide a desired field of view (FOV) for the sensor. The field of view is the area that is sensed by the sensor. Such sensor arrays may be formed from infrared (IR), electro-optic (EO), or ultraviolet (UV) types of individual sensors. Such sensor arrays typically capture images at a rate of about 100 Hz and processing circuitry in the threat launch detection system analyzes the captured images to detect a threat. These sensor arrays are relatively small and to provide a good field of view for each sensor the focal length of the associated optics must be relatively small (i.e., as the focal length decreases the field of view increases). The focal length must be kept to a reasonable value and therefore the field of view of a typical sensor array is relatively narrow, meaning that a lot of sensor arrays are needed to provide the overall field of view required by the threat launch detection system.
In operation, each sensor captures images in its corresponding field of view and the processing circuitry analyzes successive images or frames. The processing circuitry detects threats based on the differences from one frame to another. By comparing frames and analyzing in which pixel or pixels of the sensor array the threat occurred, the processing circuitry determines when the threat was fired. The term “pixel” as used herein refers generally to one of the individual sensor elements contained in a sensor array, with the sensor elements being arranged in rows and columns to collectively form the sensor array. The processing circuitry also determines the direction of detected threat from which one or ones of the sensor arrays detected the threat and the distance of the threat from the detected intensity.
These sensor arrays and the associated processing utilize what may be termed “spatial tracking” to detect threats. In spatial tracking, the pixels in a given frame are analyzed relative to the pixels in adjacent frames as just described. The position of pixels in each frame that detect some image change from frame to frame as the threat moves through space, hence the term spatial tracking. The processing in spatial tracking typically involves track processing, a form of pattern recognition as part of the detection of a threat, as will be appreciated by those skilled in the art.
These conventional threat launch detection systems utilizing IR, EO, and/or UV sensor arrays are best suited to detecting threats having relatively long durations, such as the powered fly out of a guided missile. This is true partially because the time for acquisition and processing required to analyze the frames captured by each sensor array is relatively intensive, and, as previously mentioned, numerous sensor arrays are required to provide the required overall field of view for the system. Each of these sensor arrays has numerous pixels, and the processing circuitry must separately read and analyze the data of each pixel for each array.
This intensive processing caused by the multiple sensor arrays and the large number of pixels per sensor array limits the rate at which the system can operate and thereby limits the types of threats that can be reliably detected. Short-burn threats such as tank shells or RPGs are accordingly not reliably detected by conventional threat launch detection systems. It should also be noted that a key operational characteristic of threat launch detection systems is the elimination of false detections. To do so the system typically compares three to five or more successive frames from each sensor array and analyzes the pixels to ensure the threat is present in the same pixels or pixels in each of these frames. If the threat is of sufficient duration that it is present in these pixels for successive frames then a threat is detected. If the threat is not present in each of these frames, however, such as may be the case for short-burn type threats where the threat may only be present in one or two frames, the system determines the threat is false. With these conventional threat launch detection systems, even though a real threat such as an RPG has been directed at the target containing the system, the systems have problems reliably detecting the short-burn threat.
To detect the launch of short-burn threats, conventional threat launch detection systems typically utilize IR and EO sensors operating in one or two midwave infrared bands (3-5 micron wavelength). UV sensors have also been utilized in such systems as previously mentioned. Weapons have been designed for deployment via short-burn to reduce the duration of the observability of the threat and thus prevent the system from detecting the threat. Thus, although the sensors typically “see” the threat, meaning at least some pixels in at least one sensor detect the presence of the threat, the processing of these pixels does not detect the short-burn threat.
In an attempt to more reliably detect short-burn threats, some systems have attempted to perform “temporal profiling” of the frames captured by the IR, EO and UV sensor arrays. In temporal profiling, the individual pixels are analyzed over time rather than relative to other pixels as is the case in spatial tracking. Attempts at temporal profiling have been unsuccessful for a variety of different reasons. First, as previously discussed these sensors have insufficiency of sampling rate issues, namely the rate at which the sensor arrays capture images is too slow relative to the duration of short-burn threats. Additionally, the dynamic range (i.e., the range of detectable signals from the weakest to the strongest) of these sensor arrays is insufficient to reliably detect all the various types of short-burn threats.
These sensor arrays also have loss sharing issues, meaning that the threat is detected or “shared” by multiple pixels over time. This makes reliable analysis of these pixels over time or temporal profiling extremely difficult. Even sub pixel sized threats will form images that can fall on several pixels in a sensor array due to finite optics spot size. Any line of sight motion, whether due to movement of the threat or movement of the sensor base, changes the distribution of pixels on which threat falls. This may be termed sharing noise, and this sharing noise scales with the instantaneous amplitude of the signal detected by a given pixel, making it problematic regardless of how strong the signal. To obtain accurate intensity of signal information, the processing circuitry must determine where the signal is located and then estimate the nearby background to correct for this effect. This background is difficult to accurately estimate without performing hundreds or more calculations per pixel. This calculation is useful only when the pixels can be corrected for offset and gain variation, adding a requirement that a focal plane of the sensor and associated drive electronics be very stable electronically. Those skilled in the art will appreciate that offset error decreases in significance with increased signal but gain error scales with signal input.
There is a need for a threat launch detection system and method that can reliably detect various types of short-burn threats.